feat: implement B.1 — persist login sessions in SQLite with sliding idle timeout

Sessions move from an in-memory map (absolute 12h TTL) to a `sessions`
table (migration 0002), storing only the SHA-256 of the token. Expiry is
now a sliding idle window (PANEL_SESSION_IDLE_DAYS, default 7, no
absolute cap), extended at most once an hour and never by the
monitoring screens' background polling (GET + HX-Request), so a
forgotten open tab doesn't keep a session alive indefinitely. A login
now survives a container restart or redeploy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/handlers_auth_test.go

@@ -62,7 +62,7 @@ func TestSessionTokenIgnoresTheOtherName(t *testing.T) {
 }
 
 func TestRequireAuthRejectsDuplicateCookies(t *testing.T) {
-	s := &Server{cfg: Config{CookieSecure: false}, sessions: newSessionStore()}
+	s := &Server{cfg: Config{CookieSecure: false}, sessions: newTestSessionStore(t)}
 	token := s.sessions.Create("admin")
 
 	reached := false
@@ -85,7 +85,7 @@ func TestRequireAuthRejectsDuplicateCookies(t *testing.T) {
 // Signing out has to expire the cookie under both names, or the cookie left
 // over from a pre-__Host- build stays in the browser for the rest of its life.
 func TestLogoutClearsBothCookieNames(t *testing.T) {
-	s := &Server{cfg: Config{CookieSecure: true}, sessions: newSessionStore()}
+	s := &Server{cfg: Config{CookieSecure: true}, sessions: newTestSessionStore(t)}
 	token := s.sessions.Create("admin")
 
 	r := httptest.NewRequest(http.MethodPost, "http://panel.example.com/logout", nil)